[OPENMP][CUDA]Use __syncthreads when compiled by nvcc and clang >= 9.0.

Summary:
The problems with __syncthreads() were fixed in clang >= 9.0 and the
original __syncthreads() can be used instead of the ptx instruction.

Reviewers: grokos

Subscribers: guansong, jdoerfert, openmp-commits, kkwli0, caomhin

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D63515

llvm-svn: 363807